Bumping gaia.json for 2 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/8ad1ec8d1d4d
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Merge pull request #11647 from jonallengriffin/pointer

Bug 907388 - Update gaia-ui-tests pointer, a=test-only

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https://hg.mozilla.org/integration/gaia-central/rev/856faafc6a88
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Bug 907388 - Update gaia-ui-tests pointer, a=test-only